Reincarnation Coliseum, Vol. 1 (manga)

High schooler Kouji Mikagami is painfully average in every way—except when he's playing video games! So getting summoned to a magical world and granted the ability to copy his opponent’s skills should be his way to the top. Before he gets the chance, he's thrown into the coliseum as a combat slave...for his trash-tier skill! In the midst of his despair, he realizes only one path remains open to him—beat down the strongest girls with the weakest skill and make a name for himself!